1) Field of the Invention
The present invention relates to a powdery material feeding hopper structure for feeding powdery molding materials into a plasticizing cylinder of an in-line screw injection molding machine or an in-line screw extrusion molding machine (hereinafter, referred to simply as a molding machine).
2) Description of the Related Art
FIG. 7 is a longitudinal sectional view illustrating a plasticizing cylinder of an injection molding machine having a conventional material feeding hopper structure. As shown in FIG. 7, the plasticizing cylinder 3 is in the form of a heating cylinder having a heater not shown mounted on the outer periphery thereof. The plasticizing cylinder 3 includes therein a plasticizing screw 6 which is supported rotatably and slidably longitudinally (in the axial direction, i.e., in the right to left direction of FIG. 7). The tip (a front end) of the plasticizing cylinder 3 is provided with a nozzle 3a for injecting a plasticized molten resin material into a metal mold not shown.
The plasticizing cylinder 3 has at its upper portion to the base end (the right upper portion of FIG. 7) a circular material feed port 4 allowing a communication between the interior and the exterior of the plasticizing cylinder 3, with the material feed port 4 being surrounded by an upward projecting hopper mounting portion 21. A hopper 2 is mounted to a hopper mounting portion 21 to feed a molding resin material 1 through the material feed port 4 to the outer periphery close to the base end of the plasticizing screw 6 within the plasticizing cylinder 3.
The hopper 2 is mounted to the hopper mounting portion 21 by fastening a flange 2a formed on its lower outer periphery against the top surface of the hopper mounting portion 21 with bolts (not shown) or other fastening means.
The plasticizing screw 6 has flights 6a formed around its outer periphery for delivering the material 1 forward (leftward in FIG. 7). The plasticizing screw 6 is rotated on its base end side by a motor not shown to deliver the material 1 from the hopper 2 forward while plasticizing the same. Furthermore, the plasticizing screw 6 is displaced forward relative to the plasticizing cylinder 3 by an injection cylinder not shown.
Due to the above construction, the material 1 introduced into the hopper 2 is fed through the material feed port 4 to the outer periphery on the base end side of the plasticizing screw 6 within the plasticizing cylinder 3, and then is delivered forward (toward the tip of the screw) along the grooves defined between the flights 6a of the screw 6 in the cylinder 3 while being gradually melted by a heat from the cylinder 3 and from a share heat caused by the rotation of the screw 6.
Once it is measured that a predetermined amount of molten resin (material 1) has been fed into a space on the tip side of the screw 6, the rotation of the screw 6 is brought to a stop and the screw 6 is displaced forward relative to the cylinder 3 by the injection cylinder not shown, allowing the molten resin within the cylinder 3 to be ejected through the nozzle 3a into the metal mold not shown. When the molten resin in the metal mold is cooled, a molded part in the mold is removed to complete a series of molding cycle steps.
By the way, in case of using the material 1 in the form of a powder, air contained in the powdery material 1 is compressed to increase its pressure when the powdery material 1 is delivered toward the tip of the screw 6 while being plasticized and melted together with the rotation of the screw 6 during the above molding cycle, which may possibly cause the material 1 to flow backward toward the base end (the rear side) of the screw 6 or toward the hopper 2.
At that time, if a volume of material 1 lies within the hopper 2, the resistance against the backward flow of air at the material feed port 4 increases with the result that air and the powdery material 1 may pass through a gap 11 defined between the screw 6 and the heating cylinder 3 and having a smaller resistance than at the material feed port 4 and then spout backward of the cylinder 3, allowing the material 1A to escape to the exterior.
Thus, as shown in FIG. 9 for example, in order to prevent the air and the powdery material 1 from flowing backward of the screw 6 and from passing through the gap 11 for spouting, a technique is proposed which employs a packing 13 mounted on the inner peripheral surface on the rear end side of the cylinder 3 to thereby close the gap 11 while being in contact with the outer peripheral surface of the screw 6, with a packing gland 14 serving to prevent the packing 13 from coming off. However, this technique suffers from a drawback that the packing gland 14 becomes worn earlier due to its contact with the screw 6, allowing an immediate escape of the powdery material 1.
In another technique (see Japanese Utility Model Pub. No. HEI2-34009) shown in FIG. 10 for example, an inner hopper 7 is provided on the inside of the hopper 2 by means of an engagement protrusion 9, to define a gap 18 extending from the bottom of the hopper 2 to the top thereof between the inner hopper 7 and the outer hopper 2. By virtue of such a hopper structure, as indicated by an arrow a' in FIG. 10, gas or the like within the cylinder 3 passes through the gap 18 to the exterior so that there is eliminated a variation in pressure caused by the material 1 lying in the bottom of the hopper 2, thereby making it possible to keep the feed pressure applied to the screw 6 at a certain level and to achieve a stable plasticization.
However, even in case of such a hopper structure as shown in FIG. 10, the material feed port 4 is actually at all times filled with the material 1 as shown in FIG. 10 and the hopper 2 is also loaded with the material 1, so that it is difficult to cause the backward flow of air or the like as described above to easily escape into the gap 18, eventually allowing the material to pass through the gap between the cylinder 3 and the screw 6 to spout from the rear end side.